What happens at the core of the sun?

The correct answer is Option (3) → Hydrogen → Helium

The energy of the sun comes from thermonuclear fusion in its core, where light nuclei join to form a more tightly bound, heavier nucleus.

Option (1) Helium → Hydrogen — Incorrect. This is the reverse of fusion. Helium is more tightly bound than hydrogen. So breaking it into hydrogen would need a supply of energy. It would not release any.

Option (2) Hydrogen → Deuterium — Incorrect. Two protons forming a deuteron is only the first step of the proton-proton cycle. The deuterium is used up in the next step, and the end product is helium.

Option (3) Hydrogen → Helium — Correct. In the proton-proton cycle, the hydrogen in the core is burned into helium. The net effect is

    \( 4\,{}^{1}_{1}\mathrm{H}+2e^{-}\rightarrow{}^{4}_{2}\mathrm{He}+2\nu+6\gamma+26.7\,\mathrm{MeV} \)

Option (4) Deuterium → Hydrogen — Incorrect. In the sun, deuterium is formed from protons. It then fuses with another proton into helium-3. It is never turned back into hydrogen.

NCERT (2026-27):
Nuclear Fusion – Energy Generation in Stars – “The fusion reaction in the sun is a multi-step process in which the hydrogen is burned into helium. Thus, the fuel in the sun is the hydrogen in its core.”